RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage tumor cells. It is not yet known which treatment regimen is more effective for rectal cancer.

PURPOSE: Randomized phase III trial to compare the effectiveness of different regimens of combination chemotherapy plus radiation therapy in treating patients who have rectal cancer that has been surgically removed.

OBJECTIVES: I. Compare the overall and relapse free survival of patients with stage II or III rectal cancer treated with one of the following three regimens: bolus injections of fluorouracil (5-FU) prior to and following pelvic irradiation plus protracted venous infusion (PVI) 5-FU radiosensitization vs PVI 5-FU prior to and following pelvic irradiation plus PVI 5-FU radiosensitization vs bolus 5-FU with leucovorin calcium and levamisole prior to and following pelvic irradiation. II. Describe relapse patterns and tolerance associated with these regimens in these patients.

OUTLINE: This is a randomized, multicenter study. Patients are stratified according to type of prior surgery (abdominoperineal resection vs anterior resection), nodal status (N0 vs N1 vs N2-3), depth of tumor invasion (T1-2 vs T3 vs T4a vs T4b), time from surgery to study entry (20-45 days vs 46-70 days), participating center, and performance status (0-1 vs 2). Patients are randomized to one of three treatment arms. Arm I: Patients receive fluorouracil (5-FU) IV on days 1-5 and 29-33. 5-FU is then given as a continuous infusion beginning on day 57 and continuing concurrently with radiotherapy for 5 weeks. Following a 28 day break from treatment patients receive 5-FU IV on days 1-5 of a 28 day course. Postradiotherapy treatment repeats for a total of 2 courses in the absence of disease progression or unacceptable toxicity. Arm II: Patients receive 5-FU IV continuously on days 1-42. 5-FU and radiotherapy are then administered as in arm II. Arm III: Patients receive leucovorin calcium (CF) IV followed by 5-FU IV on days 1-5 and 29-33. Patients also receive oral levamisole twice daily on days 1-3, 15-17, 29-31, and 43-45. CF IV and 5-FU IV are then given on days 57-60 and 85-88 concurrently with radiotherapy. Following a 28 day break from treatment patients receive CF IV and 5-FU IV on days 1-5 and 29-33 and oral levamisole twice daily on days 1-3, 15-17, 29-31, and 43-45 in the absence of disease progression or unacceptable toxicity. All patients receive radiotherapy 5 days per week for 5 weeks starting on day 57. Patients are followed every 4 months for 2 years, then every 6 months for 4 years, and then annually until death.

PROJECTED ACCRUAL: A total of 1,800 patients (600 per arm) will be accrued for this study.

DISEASE CHARACTERISTICS: Histologically proven stage II or III adenocarcinoma of the rectum Tumor extends through the bowel wall and into perirectal fat or soft tissue (TNM T3-4, N0, M0) Nodes are involved with tumor (TNM T1-4, N1-3, M0) Tumor completely resected en bloc with no gross or microscopic evidence of residual disease Circumferential (radial) margins of resected adherent tumors must be specifically documented free of disease (with the sole exception of extraperitoneal serosal margins) No evidence of metastasis No regional nodal metastases (metastases outside of the pelvis) that cannot be resected en bloc with the primary lesion No distant peritoneal metastases (metastases that are not a direct extension from the primary tumor) even if grossly resected (direct extension into another structure permitted) Abdominopelvic CT required unless: Bilirubin, SGOT, and alkaline phosphatase are within normal limits, AND Operative report describes liver as normal on exploration No tumors of colonic origin, i.e.: Lower edge of the tumor is below the peritoneal reflection or a portion of the tumor is retroperitoneally located (usually posteriorly) as defined by the surgeon at laparotomy OR Lower margin of the tumor is 12 cm or less from the anal verge by proctoscopic exam No prior history of rectal cancer No stage II or III cancers of the extrapelvic colon within the past 5 years Complete surgical resection at least 5 years prior to protocol registration allowed provided no other therapy was administered Synchronous modified stage I or IIa colorectal cancer (no nodal involvement or penetration through the muscularis propria) that has been completely resected allowed Registration between 20 and 70 days after the definitive surgical procedure required Chemotherapy must begin no later than day 70 following surgery Concurrent registration on protocol SWOG-9419 allowed for patients with adequate tissue samples

PATIENT CHARACTERISTICS: Age: Over 18 Performance status: SWOG 0-2 Hematopoietic: WBC at least 4,000/mm3 Platelet count normal Hepatic: Bilirubin no greater than 2 times upper limit of normal (ULN) SGOT no greater than 2 times ULN Alkaline phosphatase no greater than 2 times ULN Renal: Not specified Other: No chronic ulcerative colitis No other serious medical illness that would preclude protocol therapy No psychiatric condition that would preclude informed consent No noncolorectal malignancy within 5 years except: Adequately treated nonmelanomatous skin cancer Adequately treated carcinoma in situ of the cervix Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: No prior immunotherapy Chemotherapy: No prior chemotherapy Endocrine therapy: Not specified Radiotherapy: No prior radiotherapy Surgery: See Disease Characteristics Other: No other concurrent antineoplastic therapy